1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method fabricating a liquid crystal display device, and more particularly, to a liquid crystal display (LCD) device having wide viewing angles.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices utilize properties of liquid crystals, such as optical anisotropy and polarity, in order to display images. Since the liquid crystal molecules have a long thin structure and an alignment orientation, alignment of the liquid crystal molecules can be controlled by application of an electric field to the liquid crystals. Accordingly, the images can be displayed by changing the alignment of the liquid crystal molecules, thereby modulating light that is polarized by the optical anisotropy of the liquid crystals. Currently, active matrix liquid crystal display (AM-LCD) devices, which have thin film transistors and pixel electrodes arranged in a matrix configuration, are being developed to have high resolution and an ability to display moving images. The basic structure of a liquid crystal display panel will be described hereinafter with reference to FIG. 1.
FIG. 1 is a perspective view of a TN mode liquid crystal display (LCD) device according to the related art. In FIG. 1, a liquid crystal display panel 11 for displaying color images includes upper and lower substrates 5 and 22, wherein a liquid crystal material 14 is injected into a space between the upper and lower substrates 5 and 22. The upper substrate 5 includes a color filter 7 having a black matrix 6, a plurality of sub-color filters red (R), green (G), and blue (B), and a common electrode 18 formed on the color filter 7. The lower substrate 22 includes a plurality of pixel regions “P,” wherein each pixel region “P” includes a pixel electrode 17 and a thin film transistor “T.”
The lower substrate 22 is commonly referred to as an array substrate such that the thin film transistors “T” are arranged in a matrix configuration and a plurality of gate and data lines 13 and 15 are electrically connected to the thin film transistors “T”. In addition, the pixel region “P” is defined by a crossing of the gate and data lines 13 and 15. The pixel electrode 17 formed within the pixel region “P” is commonly formed of transparent conductive material, such as indium tin oxide (ITO).
The LCD device displays images by aligning the liquid crystal material 14 by application of a signal from the thin film transistor “T.” Thus, an amount of transmitted light is controlled according to the alignment of the liquid crystal material 14. Since the LCD device has the common electrode 18 formed on the upper substrate 5, the liquid crystals are aligned by an electric field that is formed vertically with the upper and lower substrates 5 and 22. Accordingly, the LCD device has a high transmittance and a large aperture ratio. In addition, since the common electrode 18 is formed on the upper substrate 5 and functions as a grounding conductor, the LCD device can be safe from static electricity discharge.
FIGS. 2A and 2B are partial schematic cross sectional views of a TN mode liquid crystal display (LCD) panel according to the related art. FIG. 2A demonstrates an alignment of a TN mode liquid crystal when a voltage is not supplied to the liquid crystal panel. Accordingly, a liquid crystal 14 has a positive dielectric anisotropy and has a horizontal alignment in which liquid crystal molecules are twisted to have an angle of 90° (degrees) between the liquid crystal molecule adjacent to the upper substrate 5 and the liquid crystal molecule adjacent to the lower substrate 22.
FIG. 2B demonstrates an alignment of the liquid crystal 14 when a voltage is supplied to the liquid crystal display panel. Accordingly, the twisted liquid crystal molecules 14 become re-aligned parallel to an electric field direction when the voltage is supplied to the upper and lower substrates 5 and 22. Thus, since both contrast ratio (C/R) and luminance significantly change according to a viewing angle, a wide viewing angle cannot be achieved. To overcome the problem, the pixel is divided into a normally-white mode region and a normally-black mode region, and a set of polarizers having a vertical polarizing axis to each other are formed for each region. Accordingly, the wide viewing angle of the liquid crystal display panel can be achieved by independently supplying voltages to each region for multi-compensation of the luminance.
FIG. 3 is a schematic cross sectional view of a liquid crystal display panel having a wide viewing angle according to the related art. In FIG. 3, a nematic liquid crystal layer 60 that has a twisted angle of 90° (degrees) is disposed between first and second substrates 40 and 50. A first linear polarizer 42 is formed on the first substrate 40, and second and third linear polarizers 52 and 56 are formed between the second substrate 50 and the liquid crystal layer 60. A polarizing axis of the second polarizer 52 is parallel with a polarizing axis of the first polarizer 42, and a polarizing axis of the third polarizer 56 is perpendicular to a polarizing axis of the first polarizer 42, wherein the second and third polarizers 52 and 56 are formed within a pixel region “P.” A first one-half of the pixel region “P” is defined as a normally-black mode region (NB) “A” and a second one-half of the pixel region “P” is defined as a normally-white mode region “B.” Accordingly, the second polarizer 52 corresponds to the normally-black mode region “A” and the third polarizer 56 corresponds to the normally-white mode region “B.”
FIG. 4 is a schematic plan view of FIG. 3 according to the related art. In FIG. 4, liquid crystal molecules 60a in the normally-black mode region “A” and liquid crystal molecules 60b in the normally-white mode region “B” each have twisted structures with a twist angle of 90° (degrees) when the voltage is not supplied. Accordingly, light is linearly polarized when it passes through an upper polarizer 42a, wherein a polarizing direction of the light rotates 90° (degrees) after passing through the TN mode liquid crystal. Thus, the light is intercepted within the normally-black mode region “A” where the polarizing axis of a lower polarizer 52a is parallel with the polarizing axis of the upper polarizer 42a. Moreover, the light is transmitted within the normally-white mode region “B” where the polarizing axis of a lower polarizer 56a is perpendicular to the polarizing axis of the upper polarizer 42a. As a result, the normally-black mode region “A” generates a black state and the normally-white mode region “B” generates a white state when the voltage is not supplied. Therefore, if different voltages are supplied to the normally-black mode region “A” and the normally-white mode region “B”, an average luminance value of the normally-black mode region “A” and the normally-white mode region “B” becomes a gray level and a wider viewing angle can be acquired.
FIGS. 5A to 5C are graphs demonstrating viewing angle properties of a liquid crystal display (LCD) panel having a wide viewing angle according to the related art. FIG. 5A demonstrates viewing angle properties of a liquid crystal display panel operated in a black state. In FIG. 5A, a graph portion 60 corresponds to a black state at the normally-black mode region, a graph portion 62 corresponds to a black state at the normally-white mode region, and a graph portion 64 corresponds to a black state at both the normally-black mode region and the normally-white mode region. When the black mode is at the normally-black mode region, a measured luminance is close to zero within a range of designated viewing angles between −80° (degrees) and +80° (degrees). On the other hand, when the black state is at the normally-white mode region, luminance increases as the viewing angle increases along upward and downward directions. Accordingly, from the graph portion 64, luminance is lower than the luminance of the black state at the normally-white mode region. Thus, the liquid crystal display panel having the normally-black mode region and the normally-white mode region within one pixel region can display clearer dark images than a liquid crystal display panel having only a normally-white mode region within a pixel region.
FIG. 5B demonstrates viewing angle properties of a liquid crystal display panel operated in a middle gray state. In FIG. 5B, a graph portion 70 corresponds to a luminance property of the normally-black mode region, a graph portion 72 corresponds to a luminance property of the normally-white mode region, and graph portion 74 corresponds to a luminance property of the normally-black mode region and the normally-white mode region, each within a range of designated viewing angles. As shown in FIG. 5B, the graph portions 70 and 72 are symmetric to each other about a vertical axis. Accordingly, each of the graph portions 70 and 72 shows significant luminance differences between a positive viewing angle region and a negative viewing angle region. Thus, luminance distribution on the liquid crystal display panel is not uniform. However, the graph portion 74 shows a uniform luminance property within the range of designated viewing angles. This is a result of mutual luminance compensation of the normally-black mode region and the normally-white mode region. Accordingly, gray inversion does not occur in a wide range of viewing angles.
FIG. 5C demonstrates viewing angle properties of a liquid crystal display panel operated in a white state. In FIG. 5C, a graph portion 80 corresponds to a normally-black mode region, a graph portion 82 illustrates corresponds to a normally-white mode region, and a graph portion 84 corresponds to a normally-black mode region and a normally-white mode region, each within a range of designated viewing angles. As shown in the graph portion 80, luminance abruptly decreases as the viewing angle increases along both positive and negative directions. In the graph portion 82, luminance is relatively uniform within a wide range of viewing angles. Accordingly, as shown in the graph portion 84, if the normally-black mode region and the normally-white mode region are formed within a pixel region, a relatively uniform luminance distribution can be obtained. Thus, if the normally-black mode region and the normally-white mode region are simultaneously formed within a single pixel region, a wider viewing angle can be acquired as compared to the liquid crystal display panel operated only with the normally-black mode region within a pixel region.
Accordingly, wide viewing angles can be obtained by forming a normally-black mode region and a normally-white mode region to provide mutual luminance compensation.